The formation of an insulation layer by implanting ions into a semiconductor substrate is a well-known technique as disclosed in Japanese Patent Publication No. 49-39233. In the past, as a method for implanting oxygen ions into a single crystal silicon substrate to form a silicon oxide film which is an electrically insulative material (hereinafter referred to as a buried oxide layer) in the substrate and form a single crystal silicon layer having a uniform thickness over an entire surface of the substrate on the buried oxide layer, the following methods have been known.
Japanese Patent Publication No. 62-12658 discloses a method of ion-implanting oxygen into a single crystal substrate kept at a temperature of 200.degree. C. or higher to result in a concentration which is as 1.5 times as large as an oxygen concentration (4.5.times.10.sup.22 ions/cm.sup.3) in a uniform insulation layer to form a discrete boundary between the buried oxide layer and the single crystal silicon layer on the surface. This method, however, has a defect in that the dislocation density in the single crystal silicon layer on the surface is 10.sup.5 /cm.sup.2 or higher.
JP-A-62-188239 and U.S. Pat. No. 4,676,841 disclose a method for forming a sharp boundary free from non-stoichiometric silicon dioxide at the boundary of the single crystal silicon layer and the buried oxide film by implanting oxygen ions into the silicon substrate at an energies of 100 to 400 KeV and a dose of 5.times.10.sup.17 to 5.times.10.sup.18 ions/cm.sup.2 and thermally processing it in an nitrogen atmosphere at a temperature of at least 1300.degree. C. for six hours to ten minutes. This method, however, has a defect in that the dislocation density in the single crystal silicon layer on the surface is 10.sup.3 /cm.sup.2 or higher and a continuous buried oxide film cannot be formed.
JP-A-64-17444 discloses a method for forming an oxide or nitride buried insulation layer by conducting multiple continuous implantations of oxygen or nitrogen ions into the silicon substrate at the same energy and at a dose of 1.5.times.10.sup.18 ions/cm.sup.2, and after each implantation, annealing the substrate at a temperature no lower than 800.degree. C. and no higher than a melting point of the substrate. This method, however, has a defect in that the dislocation density in the single crystal silicon layer on the surface is no higher than 10.sup.5 /cm.sup.2 but it is lower than 10.sup.3 /cm.sup.2.
JP-A-2-191357 discloses a method for preventing channeling in the oxygen ion implantation by implanting silicon ions at a dose of 10.sup.18 ions/cm.sup.2 to such a depth that the oxygen ion concentration exhibits 10.sup.21 ions/cm.sup.3 (shallower one of two) to make it amorphous and then implanting oxygen ions, and preventing the occurrence of crystal defect in a surface silicon layer which is induced by void holes or excess oxygen by reducing the void holes and excess oxygen by implanted silicon. However, this method has a defect in that the dislocation in the single crystal silicon layer on the surface rather increases due to the silicon atoms and the excess implanted silicon atoms generated in the lattices in the course of the formation reaction of the buried oxide film.
JP-A-3-240230 discloses a method for forming a thin surface single crystal silicon layer on a buried oxide film of a predetermined film thickness while preventing the generation of crystal defects by reducing the oxygen ion dose, by applying a first thermal process to a silicon substrate to which oxygen ions have been implanted at a first implantation energy, specifically applying the thermal process in an Ar atmosphere at 1320.degree. C. for six hours to form a first buried oxide film, and then implanting a smaller amount of oxygen ions than that of the first implantation at a second implantation energy lower than the first implantation energy and applying the same thermal processing as that of the first thermal processing to form a second buried oxide film superimposed on the first buried oxide film. However, this method cannot reduce the oxygen ion dose to attain the buried oxide film of the predetermined thickness, and the dislocation density in the single crystal silicon layer on the surface is 10.sup.3 /cm.sup.2 or higher.
JP-A-4-249323 discloses a method for implanting first oxygen ions into a major surface of a silicon substrate at a first high energy which imparts an oxygen concentration distribution in which the oxygen concentration is maximum at a position below the major surface of the silicon substrate, then thermally processing the silicon substrate to form a buried oxide film in the silicon substrate, then implanting second oxygen ions into the major surface of the silicon substrate at a second high energy which imparts an oxygen concentration distribution in which the oxygen distribution is maximum in a vicinity of interface between the buried oxide film and the overlying surface silicon layer, and then thermally processing the silicon substrate to make the interface between the buried oxide film and the surface single crystal silicon layer planar. However, this method has a defect in that the dislocation density in the surface single crystal semiconductor silicon layer is 10.sup.3 /cm.sup.2 or higher.
JP-A-4-264724 discloses a method for making the islocation in the surface single crystal silicon layer 10.sup.3 /cm.sup.2 or lower by implanting oxygen ions into the silicon substrate at an acceleration energy of 150 KeV to 200 KeV and at a dose of no smaller than 0.25.times.10.sup.18 ions/cm.sup.2 and no larger than 0.50.times.10.sup.18 ions/cm.sup.2 or no smaller than 0.80.times.10.sup.18 ions/cm.sup.2 and no larger than 1.30.times.10.sup.18 ions/cm.sup.2 and then thermally processing it at a high temperature of 1300.degree. C. or higher. However, as disclosed by S. Nakashima et al in J. Mater. Res., Vol. 8 (1993), pp. 523-534, this method includes the following defect: when the oxygen ion dose is no higher than 0.3.times.10.sup.18 ions/cm.sup.2, a continuous buried oxide film is not always formed and dislocations of 10.sup.3 /cm.sup.2 or higher are generated in the single crystal silicon layer on the buried oxide film. These dislocations cause a current leakage of semiconductor devices formed on the SOI structure and deteriorate the characteristic of the semiconductor devices. When the oxygen ion dose is no smaller than 0.4.times.10.sup.18 ions/cm.sup.2 and no larger than 1.2.times.10.sup.18 ions/cm.sup.2, an imperfect silicon oxide film including silicon grains is formed. The buried oxide film which includes the silicon grains has a low dielectric breakdown voltage and deteriorates the characteristic of the semiconductor devices formed on the SOI structure. When the oxygen ion dose exceeds 1.2.times.10.sup.18 ions/cm.sup.2, dislocations of 10.sup.3 .about.10.sup.9 /cm.sup.2 occur in the single crystal silicon layer on the buried oxide film. When the oxygen ion dose is no smaller than 0.3.times.10.sup.18 ions/cm.sup.2 and no larger than 0.4.times.10.sup.18 ions/cm.sup.2, the dislocation density in the single crystal silicon layer on the buried oxide film is no larger than 10.sup.3 /cm.sup.2 and the continuous buried oxide film including no silicon grain is formed but, in this case, the thickness of the buried oxide layer is limited to approximately 70 to 90 nm. In this connection, Y. Li et al discusses in J. Appl. Phys., Vol. 70 (1991), pp. 3605-3612, a critical oxygen ion dose at which the continuous buried oxide film is formed as a function of a projected range of the oxygen ions. A. K. Robinson et al discloses in Maer. Sci. Eng., B12 (1992), pp.41-45, that the oxygen ion dose at which the buried oxide film including no silicon grain is formed and the single crystal silicon layer with few dislocations is formed is 0.33.times.10.sup.18 ions/cm.sup.2 for the implantation energy of 70 KeV.
JP-A-4-737 discloses a method for simply forming a silicon layer of sufficiently thin thickness such as 80 nm on a buried oxide film of sufficiently large thickness, such as 400 nm, in which a cap film comprising a silicon oxide film or a silicon nitride film is formed on a silicon substrate, then oxygen ions are implanted into the silicon substrate through the cap film, then it is thermally processed to form a buried oxide film, and the cap film is removed before or after the thermal process. However, this method does not always allow the formation of the continuous and homogeneous buried oxide film and the single crystal silicon layer with low dislocations.
U.S. Pat. No. 5,080,730 discloses a method for forming a narrow buried oxide film by reducing an implantation energy to compensate for the erosion of the surface of the silicon substrate by the ion beam when the oxygen or nitrogen ions are implanted into the silicon substrate so that the ions are implanted into the same position of the silicon substrate, and a method for forming a wide buried insulation film by increasing the implantation energy to compensate for the erosion of the surface of the silicon substrate by the ion beam. However, this method does not always allow the formation of the continuous and homogeneous buried insulation film of a desired thickness and the single crystal silicon layer with low dislocations.